The invention relates generally to monitoring a rotating member. More particularly, the invention relates to an apparatus and method for monitoring in real time a vibration phasor at the fundamental frequency of a rotating shaft to determine, for example, whether such vibration is within a prescribed limit or range.
Electrical generating equipment, which often includes a rotating member, is often relied upon for mission critical tasks where a failure thereof can lead to increased expense or possible catastrophic effects, including machinery breakdown or even bodily injury. Thus, it is desirable to monitor this equipment in an effort to prevent such failures. Specifically, turbines are often brought up to or down from operating speed in stepped intervals to, for example, reduce thermal stress on the equipment. However, a rotating machine has natural resonant frequencies, which frequencies sometimes coincide with frequencies generated during the acceleration or deceleration process. To reduce damage to the machinery, it is desirable that these points of resonance be avoided to the extent possible during the speeding up or slowing process.
Monitoring of rotating machinery, and electrical generating equipment in particular, can be accomplished by monitoring changes in both magnitude and angle (relative to an index point on a shaft, for example) of a vibration phasor or vector. Changes outside acceptable limits can be reason to trip or halt the machinery to avoid damage or avoid further damage from occurring. Alternatively, data indicative of changes outside the acceptable limits could be used by a control algorithm to operate differently and thereby restore vibration to a within acceptable limits. Significantly, changes in magnitude and/or angle may occur rapidly such as when the machinery incurs a structural failure. Rapid vibration changes may occur as a turbine's rotating frequency passes through resonant frequencies. Vibration phasor changes may also occur slowly as the result of expected or unintended component wear. Since there is the possibility that the changes may be rapid, it is desirable, for protection to be effective, that the magnitude and angle of the vibration phasor be determined continuously in real time.
In General Electric's prior art rotating member vibration phasor monitoring methods, vibration phasor magnitude and angle are determined by post-processing via Fourier analysis of an array of readings obtained from a displacement transducer. However, the delay caused by the accumulation of the readings and data transfer from the input and output (I/O) card to, for example, a personal computer-based human machine interface for subsequent processing results in magnitude and angle updates too slow for protection from or control of rapid vibration changes and thus this method is suitable, at best, only for trending to monitor component wear. A system based on Fourier analysis is in use in, for example, General Electric's Speedtronic Mark V turbine controller.
Another vibration monitoring technique is described in U.S. Pat. No. 3,220,247 to Goodman, which is directed to detecting vibration in marine propulsion equipment. In Goodman, sine and cosine generators are provided which generate reference signals with reference periods which are the same as the periods of an unbalance signal. The unbalance signals and reference signals are coupled to multipliers and the resulting products are passed through filtering circuits to obtain average or mean values. In Goodman, a physical connection of a tachometer-generator to a rotating shaft is necessary. Such a connection, however, may be complicated and therefore costly. Furthermore, the tachometer-generator is subject to mechanical wear and might require that the machinery being monitored be shut down in the case of its failure, even though the machinery itself is experiencing no malfunction. Such unnecessary shutdowns can be extremely expensive for power plant operators and others. Further still, the 90 degree quadrature relationship of the sine and cosine references from the tachometer-generator is critical to the accuracy of any calculations. Unfortunately the 90 degree relationship relies on manufacturing tolerances in placing the respective windings of the tachometer-generator at 90 degrees from each other. Also, Goodman's device does not supply the vibration phasor angle in a form usable for automatic protection or control. The data is only available for display via an oscilloscope. Even the displayed data provides only a crude means of visually determining the angle. Additionally, the reference point on the rotating shaft to which the phasor angle is measured in Goodman is that point in shaft rotation that results in the tachometer-generator's sine output equal to 0 and cosine output equal to 1. If the coupling of the tachometer-generator to the shaft slips, the reference point on the shaft slips, i.e., moves as well. Finally, examination of harmonic vibrations in Goodman's apparatus would require a gear box or a multiple winding tachometer-generator, which adds yet further complications and expense.
Another vibration monitoring technique is described in U.S. Pat. No. 4,015,480 to Giers, which is directed to instantaneous measurement of unbalance. This apparatus includes the multiplication of the sine and cosine components of a reference phasor with multiple readings of vibration magnitude. Giers' apparatus, however, is also deficient in a number ways. The apparatus requires physical connection of the clock generator, or in the case of a physical reference generator, both the reference and clock generator, to the rotating shaft. Such a connection may be difficult to accomplish and therefore undesirable. Further, Giers' sampling frequency is dependent on the number of holes on the outer circumference of the disk in the clock generator. A high sampling frequency as desired for accurate and high resolution calculation of the phasor magnitude and angle would require an ever larger disk with more holes, which could become unmanageable. Further still, Giers' apparatus requires synchronization of the reference and clock generators and compensates for less than perfect synchronization by increasing the sampling frequency. However, sampling frequency is limited to the number of holes as discussed above.
Further still, consistent and accurate sampling frequency and period in Giers depends on accurate placement of the holes in the disk of his clock generator. This requires precision manufacturing techniques. Also, as with Goodman, examination of harmonic vibrations would require a gear box.
Thus there is a need for a simple, real-time method and apparatus for accurately and effectively monitoring a vibration phasor in a rotating member for effective monitoring and control.